The present disclosure relates generally to gas turbines, particularly, to aeroderivative gas turbines. More specifically, the present disclosure relates to devices and methods for unlocking a gas turbine, following the shut-down and rotor locking due to temperature differentials inside the turbomachinery.
Aeroderivative gas turbines are widely used as power sources for mechanical drive applications, as well as in power generation for industrial plants, pipelines, offshore platforms, LNG applications and the like.
The gas turbine can be subject to shut-down, e.g. in emergency situations, and restarted after a brief time period. When the rotor of the turbine is left motionless upon shut-down, thermal deformations may occur with the reduction or elimination of clearances between rotor and stator parts, hence leading to a rubbing between rotor and stator parts or a rising up to rotor locking phenomena. Thermal deformations are related to not uniform temperature fields, due to several factors. Cooling of the rotor when the turbine is motionless is non-uniform, the upper part of the rotor cools at a lower rate than the lower one, due to natural convectional phenomena, thus generating rotor bending and bowing deformations. Reduction of clearances between stator and rotor can also arise from temperature spreads related to the secondary flow distribution during shut-down. The turbine cannot be restarted until the rotor has reached the proper temperature field, as well as geometry. Under this respect, the most critical parts of the aeroderivative gas turbine are the blade tips in the compressor stages, where a limited clearance is provided between the stator and the rotor.
For some types of gas turbine-emergency shut-down the cool-down process requires significant amount of time, during which the turbine and the driven load can therefore not be restarted. This can cause substantial economic loss and/or create technical or management problems.
It has been suggested to solve this problem by keeping the turbine rotor revolving under a slow turning condition during the shut-down period, thus avoiding non-uniform cool-down of the rotor and preventing the latter from locking. This is usually done by driving the turbine rotor into rotation by means of the start-up electric motor. The start-up electric motor requires a large amount of electric energy to be powered. For some particular plant emergency shut-down conditions, no AC current is available, thus no start-up motor or any high energy consumption utility may be used.